This invention relates to unique transparent plastic (preferably, though not necessarily polypropylene) articles that can be tailored to become opaque when exposed to a sufficiently high temperature and which return to substantially the same transparency level upon cooling. Such formulations include non-polypropylene polymeric constituents that exhibit refractive index measurements similar to the base clarified polypropylene at lower temperatures, as well as melting temperatures well below that for the base clarified polypropylene. Upon exposure to temperatures in close proximity to the melting temperature of the non-polypropylene polymeric constituents, the refractive index for such constituents will then become modified to the extent that the overall article appears at least partially opaque. In particular, the non-polypropylene polymeric constituents should exhibit melting temperatures well below that for the base clarified polypropylene, from about 60 to about 100xc2x0 C. (well below the typical polypropylene melting temperatures of roughly about 160-190xc2x0 C. for homopolymer and about 140-170xc2x0 C. for typical random copolymer, both nucleated or non-nucleated). In this manner, a temperature sensitivity measuring thermoplastic article may be provided, and may be tailored to specific temperature ranges dependent on the melting temperatures exhibited by the non-polypropylene polymeric constituents. Methods of measuring temperature levels via the transformation of transparent polypropylene formulations to at least partially opaque versions thereof are also encompassed within this invention.
Clarified (a.k.a., transparent) polypropylenes have been utilized in a variety of end-use applications, including storage containers, medical devices, food packages, plastic tubes and pipes, shelving units, and the like. Such base compositions, however, must exhibit certain physical characteristics in order to permit widespread use. Uniformity in arrangement of crystals upon crystallization is a necessity to provide an effective, durable, and versatile polypropylene article. In order to achieve such desirable physical properties, it has been known that certain compounds and compositions provide nucleation sites for polypropylene crystal growth during molding or fabrication. For clarification purposes, such crystals must exhibit very small sizes to reduce the haze within the target article. Generally, compositions containing such nucleating compounds crystallize at a much faster rate than unnucleated polyolefin. Such crystallization at higher temperatures results in reduced fabrication cycle times and a variety of improvements in physical properties, such as, as one example, stiffness.
Such compounds and compositions that provide faster and or higher polymer crystallization temperatures are thus popularly known as nucleators. Such compounds are, as their name suggests, utilized to provide nucleation sites for crystal growth during cooling of a thermoplastic molten formulation. Generally, the presence of such nucleation sites results in a larger number of smaller crystals. As a result of the smaller crystals formed therein, clarification of the target thermoplastic may also be achieved, although excellent clarity is not always a result. The more uniform, and preferably smaller, the crystal size, the less light is scattered, as alluded to above. In such a manner, the clarity of the thermoplastic article itself can be improved. Such clarified polypropylenes are well known within the polyolefin industry and pertinent art to that effect is noted below.
Polyethylenes have been added to such clarified polypropylenes in the past in order to provide improvements in impact resistance, sometimes with very little detrimental effect on the haze characteristics thereof. However, in the past, such polyethylene additives have exhibited problematic high temperature opacifying properties thereby compromising the clarified polypropylene for certain end-uses. In particular, the low amount of impact resistance-improving polyethylenes provide amorphous characteristics upon exposure to sufficient heat, again thereby affecting the transparent nature of the target article. To date, such a problem has remained as such an undesirable issue within such polymer articles. Nowhere in the prior art has this phenomenon been further studied and improved upon for the purpose of utilizing such a past problem for certain benefits. It is the direction of this invention to investigate the possibilities of modifying such transparent polypropylene formulations into unexpectedly effective temperature indicators for certain end-uses.
Such an opacifying problem in the past is generally associated with the refractive index measurements of the component plastic phases within the particular article. When approaching or reaching the target plastics melting temperature, a sudden change in refractive index occurs. In the past, as noted above, it was noticed that certain blends of different plastics (such as the aforementioned polypropylene including strength-enhancing amounts of polyethylene) having similar refractive indices at room temperature will appear transparent, at least to some degree in visible light. Any sufficient modification of the refractive index differences between such blended plastics will result in the scattering of light at the boundary (or boundaries) of the different phases of plastic components. Such a sudden change may be caused by exposure to higher temperatures (e.g., a temperature high enough to cause at least partial melting of one the plastic phases therein), thereby causing an increase in light scattering within the target transparent article. In such an instance, the target article will then appear opaque.
In the past, such an opacification problem has proven detrimental as the consumer needs for such articles relies on retained transparency, rather than low temperature generation of opaque characteristics. However, it has now been determined that such a past problem can be controlled and tailored to a certain level in order to provide beneficial temperature sensing abilities in an effort to provide a safer, simplified guide to the consumer as to the temperature level exhibited by a target liquid, foodstuff, or other item contained within such a transparent target plastic article, or surface to which a transparent target plastic article is contacted. Such a development is not simple to achieve as the selection of proper non-polypropylene polymeric constituent(s) having the necessary room temperature refractive index levels similar to the base polypropylene, as well as the proper range of melting temperatures to provide sufficient opacity indications of temperature levels requires extensive consideration of different potential additives of this type, particularly to permit a return to substantially the same room temperature transparency level after exposure to sufficiently high temperatures to effectuate the opacity indications needed for such a temperature sensing method. Considering the desire to avoid utilization of mercury- or solvent-based thermometers, and the continued interest in protecting consumers from high temperature food and drink items (e.g., microwaveable food, hot coffee, and the like), such a simplified and safe temperature sensing method is desirable.